What size furnace do I need?
Short answer
A 2,000 square foot house with average insulation in a cold climate (zone 5) needs about 95,000 BTU of heat: a 100,000 BTU input furnace at 95% efficiency. In a warm zone 3 climate, it’s about 75,000 BTU, or an 80,000 BTU furnace.
Work it out for your numbers
Heat your home needs
95,000 BTU per hour
- Furnace input rating
- 100,000 BTU per hour
- Nearest standard size
- 100,000 BTU
Furnace size by home size and climate
| Climate | 1,500 sq ft | 2,000 sq ft | 2,500 sq ft |
|---|---|---|---|
| Zone 1–2: hot | 60,000 | 80,000 | 100,000 |
| Zone 3: warm | 60,000 | 80,000 | 100,000 |
| Zone 4: mixed | 80,000 | 100,000 | 120,000 |
| Zone 5: cold | 80,000 | 100,000 | 140,000 |
| Zone 6–7: very cold | 100,000 | 120,000 | Over 140,000 |
Nearest standard furnace input size in BTU per hour, for a 95% efficient furnace, average insulation and 8-foot ceilings.
How the estimate works
The rule of thumb is BTU per square foot by climate: roughly 30 to 35 in the hottest US zones, rising to 50 to 60 in the coldest. The calculator uses the middle of each range, then adjusts for taller ceilings and for insulation that’s better or worse than average.
Input rating vs heat output
Furnaces are sold by their input rating, the fuel they burn. Efficiency (AFUE) says how much of that becomes heat. A 100,000 BTU furnace at 95% AFUE delivers 95,000 BTU; at 80% it delivers 80,000. So the same house in zone 5 needs a 100,000 BTU high-efficiency furnace but a 120,000 BTU standard one.
Bigger isn’t better
An oversized furnace heats the house in short bursts, switching on and off often. That wastes fuel, wears parts faster and leaves rooms unevenly warm. If you’re replacing an old furnace, don’t just match its size, especially if you’ve added insulation or new windows since.
Before you buy
Use these numbers to budget and to sanity-check quotes. The final size should come from a Manual J load calculation, which a good contractor does from your actual walls, windows, ducts and air leaks.
Common questions
How many BTU do I need per square foot?
Roughly 30 to 35 in the hottest US climates, 40 to 50 in mixed and cold ones, and 50 to 60 in the coldest, for average insulation and 8-foot ceilings.
What size furnace for a 1,500 square foot house?
About 80,000 BTU input at 95% efficiency in a cold or mixed climate, and 60,000 in a warm one.
Is it bad to oversize a furnace?
Yes. It short-cycles, wastes fuel, wears out sooner and heats unevenly.
Updated October 29, 2026. Figures are estimates for planning. Check product labels and local codes before you buy or build.